Infra red photo detector systems

ABSTRACT

An infra red photo detector system comprises a piece of detector material, such as Cd x  Hg 1-x  Te, InSb, InAs, etc, carrying at least a pair of spaced electrodes. An optical arrangement directs a small spot of radiation onto the detector. The position of the small spot on the much larger detector is found by applying an electrical bias between the electrodes causing a drift of photo carriers. The bias may be of alternating polarity and the detector output measured at each polarity. Alternatively a high frequency bias may be applied and the A.C. offset from the detector used to indicate spot position. Alternatively the spot position may be modulated or swept along the detector by a mirror moving in a sawtooth scanning action.

This invention relates to infra red (I.R.) photo detector systems, usingdetectors that change their resistivity when illuminated by infra redradiation.

Photo detectors are used in a variety of infra red detection systems.For example burglar alarms where the body heat, of a person passingthrough a door, is focussed onto an infra red detector. The change inresistivity of the detector causes an alarm signal. Such a system sensesonly the near presence of a person or other hot object such as a fire.More elaborate systems may use an array of detectors to give anapproximate position or direction of a hot object in accordance with theparticular detector illuminated. Such systems are often termed staringarrays. The positional accuracy for determining hot objects is limitedby the number and size of detectors. Using very large numbers of smalldetectors a reasonable image of a thermal scene is theoreticallyobtainable. However each detector is only capable of providing a signalrepresenting average radiation received. It cannot distinguish betweenoverall illumination and point illumination. Resolution is thus limitedby detector size.

Highly detailed information of a thermal scene is obtainable usingscanned imaging systems. In these an image of an infra red scene isscanned point by point onto one or more infra red detectors. Thescanning is often by rotating polygons and oscillating mirrors, and thedetectors cooled by liquid nitrogen.

The detector output may be viewed via a television monitor and isobserved in detail similar to that from conventional television cameras.These systems give sufficient detail for example to detect human tumoursby the temperature difference between healthy and tumerous tissues. Thehigh performance is unnecessarily good for some applications, also thecost and mechanical complexity is high.

The present invention provides greater information than is obtainablefrom known staring systems without the complexity of mechanical scanningsystems.

According to this invention an infra red photo detector system comprisesa photo detector formed by a piece of detector material between at leasttwo spaced electrodes, means for applying an electric field to thedetector, an optical arrangement for causing infra red radiation to beincident on the detector, means for measuring an electrical signaldeveloped between the electrodes while a small area of detector materialis illuminated with a spot of infra red radiation, whereby the positionof the incident spot relative to the electrodes may be determined.

The detector material may be the ternary alloy Cd_(x) Hg_(1-x) Te and ispreferably cooled e.g. by liquid nitrogen. Alternatively the detectormaterial may be InSb, InAs, etc.

The detector may be incorporated into a bridge circuit.

The optical arrangement may incorporate a mirror which is oscillated tosweep the spot in a circle on the detector.

The spot position may be determined for example by applying analternating bias to the electrodes; by scanning the spot along thedetector; applying a small modulation of spot position on the detector;or a high frequency bias to the electrodes.

The system may use a single detector or a multiplicity of detectorsforming an array.

The invention will now be described, by way of example only, withreference to the accompanying drawings of which:

FIG. 1 is a view of an infra red detector system that can operate in astaring or a swept mode;

FIG. 2 is a circuit diagram showing the detector of FIG. 1 connected ina bridge network;

FIGS. 3, 4 are graphs showing the output of the bridge for positive andnegative bias applied to the detector of FIG. 1 as a function of theposition of a small spot of illumination;

FIG. 5a is a graph of mirror position caused by scan in a saw-toothedmanner;

FIG. 5b is a graph showing the bridge output resulting from the scanningof FIG. 5a;

FIG. 5c is a block diagram of the apparatus used for scanning as shownin FIG. 5a;

FIGS. 6, 7 are views of detectors having a plurality of electrodes sothat azimuth and elevation positional information is obtainable;

FIG. 8 is a plan view of a detector showing contours of constantsensitivity for a field applied between opposing parallel electrodes.

As seen in FIG. 1 a detector system 1 comprises a piece of infra reddetector material 2 such as the ternary alloy Cd_(x) Hg_(1-x) Te.Typical dimensions are 100 μm square. Strip electrodes 3, 4 are attachedalong two opposite edges of the detector material 2. The detector 2, 3,4 is mounted inside an evacuated envelope 5 above an infra redtransparent window 6 e.g. of germanium and cooled by liquid nitrogen 7.

A mirror 8 and lenses 9, 10 direct infra red radiation onto the detectormaterial 2. The mirror 8 may be fixed, or oscillated as described later.

The detector of FIG. 1 is connected in a bridge network 11 as shown inFIG. 2. This bridge 11 comprises two fixed resistors R₁, R₂, and avariable resistor R₃ for balancing. Input to the bridge 11, either D.C.or A.C. is from a voltage supply 12.

When a spot area 13 of the detector 2 is illuminated with infra redradiation photo carriers are generated. Let the detector length beweenelectrodes 3, 4, be 2a and the position of the illuminated spot measuredfrom the centre be x, FIGS. 3, 4. For n-type material with a carrierconcentration of about 5×10¹⁴ cm⁻³ applying a positive voltage toelectrode 3 causes a photo carrier drift to the right. When the appliedfield is sufficiently high e.g. 30 volts/cm the minority carriers areswept towards the earth contact 4, where they recombine. For steadystate conditions the change of resistance between electrodes 3, 4 isproportional to the distance photo carriers drift before recombining.The part of detector material carrying the drifting carriers is ofreduced resistance compared to the remainder of the detector material.FIG. 3 shows the resistance change between electrodes 3, 4 for varyingvalues of x, the position of constant spot 13 illumination. At the righthand edge carriers recombine into electrode 4 as they are formed, i.e.zero drift path length, and so the resistance change developed is zero.At the left hand edge the carriers drift the whole length of detector,i.e. drift path length is 2a, and the resistance change betweenelectrodes 3, 4 is at a maximum value of A_(MAX).

Similarly when a negative potential is applied to electrode 3 thecarriers drift to the left FIG. 4. The minimum resistance change,developed occurs when the illuminating spot 13 is by the left handelectrode 3, and the maximum resistance change B_(MAX) occurs when thespot 13 is near the right hand electrode 4.

The position, x, along the detector 2 of illuminating spot 13 isdetermined by measuring A, B, as follows: ##EQU1##

A and B are regarded as positive quantities.

For example if the change in the negative direction, equals the changein the positive direction (B=A) then x=0, the centre.

Thus by applying first a positive potential and then a negativepotential to electrode 3 the position of the illuminating spot 13 isobtained. For example the position of a 5 μm diameter spot along a 50 μmlength is obtained.

The responses shown in FIGS. 3, 4 are idealized. In practice the curvesare not quite linear in the center region and may differ considerablynear the contacts 3, 4. Corrections for these variations may be allowedfor using a look up table e.g. using a programmed read only memory(R.O.M.)

The bridge of FIG. 2 may be adjusted to give zero output for zero pointillumination, or zero uniform illumination. When balanced for zeroillumination, uniform illumination over the complete detector frombackground radiation will give A=B corresponding to an apparent x=0.When balanced for uniform background illumination A=B=0. Deviation ofbackground will cause a spurious signal corresponding to an apparenttarget at x=0 because the equations for x above do not apply to uniformillumination. Thus continuous monitoring of background is desirable andthe bridge balanced accordingly.

The effect of background illumination and bias pedestal can be removedby scanning the spot 13 along the detector 2 with the mirror 8. FIG. 5ashows a graph of mirror 8 position which is caused by scan in asaw-toothed manner. As the mirror scans it sweeps a spot 13 of infra redradiation along the detector 2 in a direction opposite to photo carrierdrift. This results in a ramp response on the bridge output as shown inFIG. 5b. The position of this ramp signal within each scan representsthe angular position of the target forming the spot 13 relative to thescan extreme positions. The length of the ramp signal is a function ofdetector length and system optics. Target angular position is thusdetermined by a timing circuit which times the interval Φ between asynchronization pulse at the beginning of each scan and receipt of theramp signal (leading or trailing edge).

Such an arrangement is shown in FIG. 5c.

The above arrangement can be extended to determine the angular positionof a plurality of infra red sources. Each source will be detected as aramp voltage by the bridge.

In an alternative arrangement the mirror 8 is caused to precess orwobble so that a spot 13 moves in a small circle on the detector 2. Thisgives a bridge output signal

    dA=kdx.

The mean signal intensity is

    A=k(a-x)

from which x is extracted. The modulation dA is insensitive tobackground or bias pedestal.

In both the scanning and wobbling system the mirror movement is slow incomparison with carrier drift velocity. For example carrier lifetime maybe 1 to 10 μs, drift velocity 5×10³ to 5×10⁴ cm/sec depending on biaspedestal 2 to 20 ma, scan velocity 1/10×drift velocity, modulationvelocity 1/10 drift velocity.

Another form of the invention uses a high frequency bias applied by thevoltage sources 12. This is arranged such that

frequency f≳μE/2a

μ is typically 300 cm² /voltsec.

E is peak field

2a is distance between contacts 3, 4.

With such a high frequency bias carriers are not swept out within eachhalf cycle but are swept alternately towards contacts 3, 4 on alternatehalf cycles and reach the contacts 3, 4 by diffusion.

For a spot 13 in the center of the detector 2 carriers reach bothcontacts 3, 4 equally by diffusion. When the spot 13 is displaced awayfrom the centre then carriers are preferentially lost at the nearestcontact. This leads to an asymmetry in the output (A.C.) signal at theelectrodes 3, 4. The amount and direction of A.C. offset indicates spotposition x on the detector 2, and the A.C. value indicates spot 13intensity.

Alternatively the A.C. offset may be adjusted to zero by applying a D.C.bias feedback to the voltage supply 12. This optimizes sensitivity bymaximizing carrier left time over complete cycles. The spot position isrelated to the amount of D.C. bias applied.

The above systems provide only positional information x. To provide bothazimuth and elevation position the detector may be provided with aplurality of opposed electrode pairs as shown in FIGS. 6, 7.

In FIG. 6 a detector 2 is arranged as a disc with discrete pairs ofopposing electrodes 20, 20' to 25, 25'. Similarly a square shapeddetector 2 has discrete opposing electrodes 30, 30' to 34, 34' and 35,35' to 39, 39'.

Each opposing electrode e.g. 20, 20' is connected in sequence into abridge 11 while the remaining contacts remain disconnected, at afloating potential. Treating the information from each pair of contactsas previously described indicates both r, and θ information in FIG. 6.To obtain x, information in FIG. 7 bias is applied between electrodes30-34 and 30'-34' and measurements proceed as in FIG. 1, electrodes35-39 and 35'-39' remain unconnected.

Similarly to obtain y information bias is applied between electrodes35-39 and electrodes 35'-39' while electrodes 30-34 and 30'-34' areunconnected.

Another method of determining x, y uses the arrangement of FIG. 1. Theeffect of applying bias to electrodes 3, 4 is to produce in the detector2 variations in sensitivity as shown in FIG. 8 which indicates contoursof constant sensitivity. As shown these increase towards electrode 3 butreverse with reverse applied bias. The contours are symmetric about alengthwise axis X X.

The mirror 6 is caused to wobble thus sweeping a spot 13 of radiation ina circle 14. The effect of sweeping the spot 13 in a circle is to causea periodic fluctuation in the detector response as measured at theelectrodes 3, 4. For a spot along the line XX the maximum responseoccurs when the spot is nearest electrode 3. This position may be takenas a zero phase position of the mirror 8. For spot 13 incident on thedetector 2 above or below the line XX the maximum detector responseoccurs where the circles 14 tangent is parallel to a contour line andnearest to electrode 3. This maximum response occurs at some mirrorphase angle ±φ. The measurement of x and φ uniquely indicates theposition of a circle 14 (spot 13) on the detector 2. Thus information onx and φ may be fed into a look-up table formed in a random access readonly memory RAROM. The resultant output is thus the x, y position of aspot 13 on the detector 2.

I claim:
 1. An infra red photo detector apparatus comprising:aphotoconductive element having a photosensitive region located betweenat least one pair of bias electrodes; optical means for imaging a lightsource in a scene on to the photosensitive region; biasing meansarranged to provide a DC bias to the photoconductive element and providean output signal therefrom; means for scanning the light source imageposition on the photosensitive region at a rate which is slow comparedto photo-carrier drift velocity and in a direction opposite tophoto-carrier flow direction; and means for timing the element outputsignal modulation produced in response to and as a function of lightsource image scan position.
 2. An infra red photo detector apparatuscomprising:a photoconductive element having a photosensitive regionlocated between at least one pair of bias electrodes; optical means forimaging a light source in a scene on to the photosensitive region;biasing means arranged to provide a D.C. bias to the photoconductiveelement and provide an output signal therefrom; means for causing thelight source to precess on the photosensitive region at a rate which isslow compared to photocarrier drift velocity; and means for measuringelement output signal precession-induced modulation to provide anindication of light source image position.
 3. An infra red photodetector apparatus comprising:a photoconductive element having aphotosensitive region located between a plurality of opposed pairs ofbias electrodes regularly spaced around said photosensitive region;optical means for imaging a light source in a scene onto thephotosensitive region; biasing means arranged to bias each opposed pairof bias electrodes in succession; output signal controlling meansarranged to effect production of photoconductive element output signalsreflecting directional element interelectrode resistance, saidcontrolling means being operative to vary any one of the bias polarityand image position on the photosensitive region; and output signaldetecting means arranged to indicate light source image position fromelement resistance directionality.
 4. An apparatus according to claim 3wherein the photosensitive region is circular.
 5. An apparatus asclaimed in claim 3, wherein said controlling means includes a mirrorarranged to provide a saw tooth image scan.
 6. An apparatus as claimedin claim 5, wherein said output signal detecting means includes meansfor timing photoconductive element output signals relative to mirrorscanning.
 7. An apparatus as claimed in claim 3, wherein saidcontrolling means is a high frequency AC bias source and said biasingmeans comprises connections from said bias electrodes to said highfrequency AC bias source.
 8. An apparatus as claimed in claim 7, whereinsaid output signal detecting means includes means for indicating elementoutput signal offset in magnitude and direction.
 9. An apparatus asclaimed in claim 3, wherein said detecting means includes a bridgecircuit to which said photoconductive element is connected as one arm,said bridge circuit having a variable balancing resistor as a secondarm.
 10. An apparatus as claimed in claim 3, including detector coolingmeans for cooling said apparatus.
 11. An apparatus as claimed in claim3, wherein the detector material is selected from any one of the groupcomprising Cd_(x) Hg_(1-x) Te, InSb and InAs.